In the poem, death is personified. In the first stanza, the speaker says that Death "kindly stopped for [her]" in his "Carriage," which held only the two of them—the speaker and Death—and the prospect of "Immortality" (presumably the soul's eternal life). In other words, the speaker feels that she was a bit too busy living her life, so Death, like a suitor or someone who would come to woo her, graciously came to her rather than waiting for her to come to him.
Death "slowly drove" the carriage and treated the speaker with "Civility," or courteousness and friendliness. In the carriage, the speaker and Death pass a school where children are playing outside and some beautiful farm fields, and they even watch the "Setting Sun" together. It almost sounds like a date!
As it gets cooler, the speaker becomes aware that she is improperly dressed for the "Chill," so Death chivalrously takes her to a "House" that seemed like just a small "Swelling of the Ground," presumably her grave. The effect of this personification, describing Death as though it were like someone who is courteous and civil, even desirous of pleasing, renders the idea of death much less frightful than it typically is. We often think of death as something that could happen at any moment, whether we are prepared or not; and because death seems so unknowable to those of us who have not experienced it, it can feel scary. However, death, in this poem, is no scarier than a date with a nice person who seems pretty calm and relatively friendly. It seems much more familiar personified in this way.